In conventional oscillating mills, the material to be milled is ground between a rotating rotor assembly and a sieve. The desired properties of the milled material, such as particle grain size and particle flow rate, can be obtained by adequately selecting appropriate milling parameters such as the rotation speed of the rotor assembly and/or the oscillation amplitude and frequency in the case rotor assembly is oscillated. The correct selecting of appropriate milling parameters is also critical in avoiding important rise in temperature that could be detrimental to the quality of the milled material. During most milling operation, however, it can be difficult to select milling parameters that are appropriate during the whole milling operation. Indeed, during milling, the material can change its properties, for example due to increased temperature and/or humidity, rendering the milling parameters inadequate. For example, the milling parameters initially selected can cause the milling device to effectively mill the material to a given grain size after a give milling time, but the milled material having too large a grain size non-uniformity. To obtain acceptable grain size uniformity may require changing the milling parameters at that time of the milling operation. With such conventional milling device, however, the milling parameters cannot be simply changed during the milling operation.
In document WO2008028870, a milling device, particularly a disk vibration mill comprising a grinding gear and an oscillating drive, with which the grinding gear can be excited to the vibrations depending on the driving rotational speed. In order to improve the milling efficiency, the driving rotational speed during operation of the vibration mill (1) is varied in a predetermined manner. In U.S. Pat. No. 4,603,816, an attritor for solids comprises a cylindrical milling vessel comprising a rotor which rotation velocity can alternately pass from a relatively high speed to a relatively low speed.
In U.S. Pat. No. 4,603,816, an apparatus for shredding waste material includes first and second cooperating shredding rotors rotatably mounted adjacent to one another in a shredding chamber. The rotors are oscillated out of phase with one another and a crank drive arrangement allows for varying the rotational amplitude of the oscillation movement.
Document CH348599 discloses an apparatus for grinding hard grains and comprising a screen, a rotor forming a cavity to be fed with material for grinding. The apparatus further comprises a crank mechanism which reciprocates the rotor in a circular oscillatory motion with respect to the screen.
In the above cited prior art, the change in motion, such as rotational speed and/or rotational amplitude, cannot be varied according to the properties of the material being milled.